A disk drive is a non-volatile mass storage device for use in conjunction with a computer system. In this regard, disk drives are capable of storing relatively large amounts of information, such as computer programs and user data, for use on demand by the computer system. Disk drives generally include at least one disk for storing information, a spin motor for rotating the disk at a substantially constant angular speed, at least one transducer for use in transferring information between each disk surface and an exterior environment (e.g., a host computer), an actuator assembly for supporting each transducer in proximity to the corresponding disk during operation of the disk drive, a voice coil motor motor for imparting motion to the actuator assembly in response to a control signal, and a servo control unit for generating the control signal for the voice coil motor motor to controllably move the transducer with respect to the disk.
During operation of the disk drive, the at least one disk is rotated about an axis at a substantially constant angular speed. If the disk drive includes multiple disks, they are all axially aligned and rotated at the same speed. To transfer information from the exterior environment to a track on one of the disks (i.e., to perform a write operation), an appropriate transducer is first centered above the desired track and then a write signal is delivered to the transducer to cause a corresponding change on the track. Similarly, to transfer information from a desired track to the exterior environment, an appropriate transducer is centered above the desired track where it senses the information stored on the track to produce a read signal indicative of the information stored on the track.
The servo control unit is used to center the appropriate transducer above the desired track. The servo control unit receives information indicating the present location of the transducer and the desired location of the transducer. The servo control unit then uses this information to create an error signal which serves as the voice coil motor control signal for the motor. The information indicating the present location of the transducer is generally read by the transducer from at least one of the disk surfaces in the disk drive. The information may be stored on the actual disk surface which is being accessed or it may be located on another disk surface in the system. That is, multiple methods exist for storing servo positioning information on the disks of the disk drive.
One technique for storing servo information in a disk drive is known as dedicated servo. In a system using dedicated servo, all of the servo information is stored on a single disk surface in the drive. For example, in a two disk dedicated servo system having four disk surfaces, one of the disk surfaces is dedicated to servo information and the other three surfaces contain only user information. The servo information on the one servo surface is used to control the positioning of transducers relative to the other three surfaces. Dedicated servo systems performed adequately when relatively low track densities were being used in the industry. However, as track densities increased over time, it was found that thermal drift produced too much positional variance from transducer to transducer to successfully implement the dedicated servo technique.
Another technique for storing servo information in a disk drive system is known as sectored (or embedded) servo. In a disk drive system using sectored servo, each track on each disk surface in the system includes a plurality of servo sectors containing the required servo information. As the transducer reads/writes user information from/to a desired track, the servo sectors on the track are periodically sampled to provide the required position information to the servo control unit. Because the transducer that is doing the reading/writing of user information is the same transducer that is reading the servo information, thermal drift is not a problem. However, if the servo sector sampling rate being used is too low, the system's bandwidth and disturbance rejection capabilities can be severely limited resulting in poor track following and runout disturbance rejection capabilities.
In an effort to improve the servo sampling rate of a disk drive system, while still maintaining resistance to negative thermal drift effects, hybrid systems were developed that used a dedicated servo surface in conjunction with servo sectors on the other surfaces in the system. In this way, thermal effects could be rejected and a continuous stream of servo information was still available to the servo system. Hybrid systems generally perform well; however, these systems contain a relatively large amount of overhead because a fully dedicated servo surface is required in addition to the servo sectors on the data surfaces.
Therefore, a need exists for a servo technique that produces a relatively high servo sampling rate, that is not significantly affected by thermal drift, and that requires less overhead than prior hybrid servo techniques.